Polycarbonate resin is widely used for electric and electronic parts, OA instruments, household products and building material, due to excellent impact resistance, heat resistance and electric properties. Polycarbonate resin has higher flame retardancy than polystyrene resin. Some fields, particularly the fields of electric and electronic parts and OA instruments, require high flame retardancy and improvement thereof is attempted, by adding various flame retardants. For example, conventionally, organic halogen compounds or organic phosphorous compounds have been added. However, many organic halogen compounds and organic phosphorous compounds are problematic in terms of toxicity and particularly, organic halogen compounds have the problem of producing corrosive gas when burning. Therefore, demands for achieving flame retardancy by a non-halogen or non-phosphorous flame retardant have been increasing in recent years.
As a non-halogen or non-phosphorous flame retardant, use of a polyorganosiloxane compound (also referred to as silicone) has been suggested. Conventionally, as a method for obtaining an flame-retardant resin composition using a polyorganosilxane compound, known are the method of obtaining flame-retardant resin by kneading a silicone resin comprising monoorganopolysiloxane with a non-silicone polymer (JP-A-54-36365), the method of obtaining a flame-retardant resin composition by dispersing a silicone resin, prepared by mixing 100 parts by weight of polyorganosiloxane and 10 to 150 parts by weight of a silica filler, in a thermoplastic resin (JP-A-8-113712) and the method of obtaining a flame-retardant resin composition by adding a silicone resin, which has a weight average molecular weight of at least 10,000 to at most 270,000 and is soluble in a solvent, to a non-silicone resin containing an aromatic ring (JP-A-10-139964). However, although silicone resin is recognized to have the effect of imparting flame retardancy, the effect is insufficient. When the amount of silicone resin is increased to compensate this insufficiency, impact resistance of the resin composition becomes poor and there is the problem that obtaining a flame-retardant resin composition excellent in both flame retardancy and impact resistance is difficult.
As a method for obtaining a flame-retardant resin composition having impact resistance using a polyorganosiloxane compound, known are the method of obtaining a flame-retardant resin composition by compounding a composite rubber flame retardant, obtained by graft polymerizing a vinyl monomer to a composite rubber comprising polyorganosiloxane rubber and polyalkyl(meth)acrylate rubber, in a thermoplastic resin (JP-A-2000-17029), the method of obtaining a flame-retardant resin composition by compounding a polyorganosiloxane flame retardant, obtained by grafting a vinyl monomer to composite particles of polyorganosiloxane having an aromatic group and a vinyl monomer, in a thermoplastic resin (JP-A-2000-226420), the method of obtaining a flame-retardant resin composition by compounding a graft copolymer containing polyorganosiloxane, obtained by graft polymerizing a vinyl monomer to polyorganosiloxane particles of at most 0.2 μm, in a thermoplastic resin (JP-A-2000-264935) and the method of obtaining a flame-retardant resin composition by compounding respectively a specific amount of a graft copolymer containing polyorganosiloxane, obtained by graft polymerizing a vinyl monomer to polyorganosiloxane particles of at most 0.2 μm, and polytetrafluoroethylene in a thermoplastic resin (JP-A-2002-348453). However, although each flame-retardant resin composition has impact resistance of a satisfactory level, flame retardancy of a sufficient level cannot be obtained in a thin molded article, for which demands are increasing in the market, and significant improvement in flame retardancy is desired.
Usually, in emulsion polymerization of cyclic siloxane under the condition of an acid catalyst, polymerization conversion ratio is known to be approximately 85% and improvement in polymerization conversion ratio is desired (see for example U.S. Pat. No. 532,471 or “Silicone” published by The Nippon Kogyo Shimbun, Ltd., Feb. 2, 1974, p. 38).
Also, in recent years, pollution due to residue of siloxane of low molecular weight in polyorganosiloxane resin and influence of cyclic siloxane of low molecular weight to the environment is a problem and keeping the residue of low molecular weight siloxane as low as possible is becoming necessary.
On the other hand, as a method of emulsion polymerization of polyorganosiloxane, known is the process for preparing an emulsion of polyorganosiloxane obtained from polyorganopolysiloxane and organosilane having a condensation reactive group, which can be bonded with a silicone atom, or a partially hydrolyzed condensate thereof.
For example, known are the method of emulsion polymerizing cyclic organosiloxane and organotrialkoxysilane bonded with a functional group (JP-B-56-38609) and the method of adding alkoxysilanes having 3 to 4 functional groups as a crosslinking agent when polymerizing siloxane (JP-A-60-252613). Also, in another document (JP-A-61-106614), described is a process for preparing an emulsion of polyorganosiloxane, which comprises adding alkoxysilanes having 2 to 4 functional groups as a crosslinking agent in the presence of organopolysiloxane. More specifically, described is the process for emulsion polymerizing cyclic or linear siloxane of low molecular weight and a crosslinking agent, in the presence of 1 to 50% by weight, preferably 10 to 30% by weight, of polysiloxane as a pre-polymerized latex.
The above methods are methods for coexisting compounds having a hydrolyzable silicone functional group (alkoxysilane) excluding cyclic siloxane in emulsion polymerization. By coexisting multi-functional alkoxysilane as a component of siloxane having low molecular weight when polymerizing polysiloxane, unwanted condensation reaction among the crosslinking agent progresses when polymerizing and by-products such as scale particles are produced. As a result, there is concern that the properties may be adversely affected. Also, polymerization conversion ratio is insufficient and a method for improving polymerization conversion ratio further is desired.